1. Field of the Invention
This invention relates to the plugging of the more permeable strata of subterranean formations of nonuniform permeability. More particularly, the invention relates to an improved method for forming a plug covering a substantial areal extent of the more permeable strata. The method of this invention is especially useful in promoting more uniform fluid injection patterns, such as are desirable in the enhanced recovery of petroleum by displacement with flood water, and in achieving water shutoff in producing wells.
2. Description of the Prior Art
Since only a portion of the oil contained in a petroleum reservoir can be recovered by primary methods, it has become conventional practice to employ various enhanced recovery techniques to produce additional quantities of oil not economically recoverable by primary methods. Of the various enhanced recovery methods available, one of the most widely practiced techniques is the displacement of oil from the reservoir with a driving fluid such as flood water injected for that purpose. Normally, in carrying out the flooding process, a series of input wells spaced apart from one or more producing wells are drilled into and opened to the oil-producing strata. The injection well locations with reference to the production wells are selected to afford a desired flood pattern, the selected pattern depending in part upon field conditions, the location of existing wells, and the operator's preference. Aqueous drive fluid, such as water, brine, or a viscous aqueous polymer solution, is forced into the input wells under pressure, and out into the surrounding oil-bearing strata towards the producing well or wells. While waterflooding has been rather widely practiced in recent years, it is not without considerable operating problems and economic limitations, particularly those associated with low oil recoveries in proportion to the amount of water injected. Various surfactant and solvent floods have been proposed as means for recovering additional quantities of oil over that recoverable by conventional waterflooding. However, these processes face serious operating problems when practiced in heterogeneous formations containing strata or channels having permeabilities substantially higher than the bulk of the formation.
One of the major problems encountered in a flooding operation is breakthrough of the flooding medium from the flood front to the producing well relatively early in the displacement process, andd rapidly increasing producing water/oil ratios following the initial breakthrough. These difficulties result from the displacing medium channeling or fingering through the oil-bearing structure to the producing well, thus bypassing large zones of the oil-bearing strata. The reason for the channeling of the flooding medium to the producing wells and the resulting low oil recovery is due, in part, to the peculiar structure of the oil-bearing strata. Underground oil reservoirs, in most cases, consist of layers of sand or rock and, since no reservoir rock is perfectly uniform in composition and structure, the permeability will vary across the rock face or stratum. Also, fractures, cracks, vugs and other anomalies can promote channeling of the displacement fluid.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid builds up in a wide bank in front of the driving fluid which moves uniformly towards the producing well. To keep this bank of oil intact, and constantly moving towards the producing well, a substantially uniform permeability must exist throughout the strata. If this uniform permeability does not exist, or is not provided, the flooding fluid will seek the areas of high permeability, and channeling occurs with the consequent loss of some driving fluid energy and the appearance of excessive amounts of driving fluid in the producing well. Moreover, as the more permeable strata are depleted, the driving fluid has a tendency to follow channels and further increase consumption of the flooding media to the point where the process becomes uneconomical. This maximum limit, in terms of recovered oil, can be as high as 100 barrels of driving fluid per barrel of oil. It is of course desirable to operate at much lower driving fluid to oil ratios, and preferably not more than 15, and normally a maximum of 5 to 10 barrels of driving fluid per barrel of recovered oil is considered an acceptable operating condition, particularly where the driving fluid is a low cost agent such as flood water.
While a uniform flood front with reduced fingering can be obtained in some formations with a drive fluid rendered more viscous by the addition of various water-soluble polymers, or other materials capable of imparting higher viscosity to the flood water, viscous waterflooding is often ineffective in formations having severe heterogeneity as the mobility of the flood water cannot be economically reduced sufficiently to prevent or substantially restrict channeling of the flooding medium. Also, smaller volumes of aqueous polymer solution are sometimes injected into a heterogeneous reservoir to divert subsequently injected flooding media into the less permeable strata. However, such treatments are often ineffective or only partially effective in highly stratified formations. Hence, need exists for an improved flooding process for use in heterogeneous formations that will minimize channeling of the flooding medium or bypassing of large areas of the formation, or for a method for improving the effectiveness of the conventional selective plugging processes to obtain these goals.
Another problem associated with the production of oil from oil-bearing formations containing highly permeable water channels communicating the production well with a water zone is the intrusion of water into the well. Not only does this water intrusion cause production and disposal problems, but more importantly the beneficial effect of the natural water drive is at least in part lost thereby adversely affecting oil recovery. It is advantageous to at least partially plug the more permeable water channels so as to render the formation more uniformly permeable and to increase the sweep efficiency of the water drive, or alternatively to shut off the water intrusion. Heretofore, selective plugging of these water channels has presented great difficulty, it being necessary to effect at least a partial water plug without adversely affecting the permeability of the formation for oil production.
Thus, many processes have been proposed for plugging high permeability and/or water-producing zones wherein there is injected into formations containing such zones a wide variety of viscous solutions and/or plug-forming solutions. However, in many instances adequate plugging of such zones has not been achieved.
U.S. Pat. No. 3,386,509 to Froning describes a method for forming a plug at both ends of a high permeability zone between two wells wherein a sodium silicate solution is injected via one well and a gelling agent, such as carbon dioxide, is injected via the other well. The first-injected portion of the sodium silicate solution may contain little, if any, activator, such as ammonium sulfate or sodium bicarbonate. Thus, the first-injected portion forms a plug by reacting with the gelling agent with which it comes in contact in the formation. The later-injected portion contains sufficient activator to form a gel within a short time.
U.S. Pat. No. 3,396,790 to Eaton discloses a method for plugging a formation comprising: first injecting a slug of an aqueous solution containing a first reactive chemical, such as sodium silicate and a polymer or gum thickener; next injecting a slug of an inert spacer liquid; and finally injecting a slug of an aqueous solution of a second ractive chemical, such as ferrous sulfate.
U.S. Pat. No. 3,741,307 to Sandiford et al. shows a method for plugging the more permeable channels of a heterogeneous formation wherein there is injected into the formation: first a slug of a dilute aqueous solution of a water-soluble polymer thickener; and next a slug of a liquid agent that reacts in the formation to form a plugging material, such as an aqueous solution of sodium silicate, plus a gelling agent, such as ammonium sulfate, that causes a delayed gelling of the sodium silicate solution.
In spite of these and the many other known variations in methods of forming plugging precipitates in porous formations using alkali metal silicates, there remain problems in forming these plugs at the desired location in the formation. This is especially true where the nature of the formation is such that treating fluids injected through one well have only a relatively short residence time in the formation before being produced at an offset well.
Accordingly, a principal object of this invention is to provide a method for controlling the permeability of a subterranean formation of nonuniform permeability.
Another object of the invention is to provide a method for reducing channeling of a flooding medium from an injection well to a producing well via a high permeability channel.
Still another object of the invention is to provide a method for selectively plugging water-producing strata in a subterranean formation.
A further object of the invention is to provide a method for achieving a rapid set of a plug-forming solution following injection of the plug-forming solution into a formation.
A still further object of the invention is to provide a method for forming a plug over a large areal extent of the more permeable strata of a subterranean formation.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.